Some people say that black tends to spark better, sell it for firelighting as 'rare and high quality', but I have used all sorts without any noticeable differences IMHO.I got my pyrite from Ebay. If you do a google picture search you'll see what it looks like.
Black and white flint tends to knap well and grey flint poorly.
Here is another way I've seen, although it seems to use a lot of charcloth.
[video=youtube;tSwLly61QJs]https://www.youtube.com/watch?v=tSwLly61QJs[/video]
Keith does not use char cloth he uses mostly punk wood and some natural tinder.